The term "silicones" covers a broad spectrum of low viscosity liquids, oils and greases, rubbers and resins. Commercially produced silicones have become widely used in industrial sectors ranging from the pharmaceutical and cosmetic industries to the metal manufacturing industry and the electrical industry. Their broad range of commercial application stems from the different characteristics of a large number of different chemical building blocks. These chemical building blocks are known as chlorosilanes. The most significant production route to the production of silicones is the direct synthesis of methyl chlorosilanes. The direct synthesis is described in Silicones-Chemistry and Technology, CRC Press, 1991, pages 1-11 and in U.S. Pat. No. 5,264,084.
The direct synthesis of methyl chlorosilanes from silicon and methyl chloride at 250.degree. to 300.degree. C. by means of copper catalysts produces, in addition to the methyl-chlorosilanes of the general formula Me.sub.x SiCl.sub.4-x, x having values of from 0 to 4 and Me represents a methyl group, also ethylchlorosilanes, various silanes, in particular Me.sub.y HSiCl.sub.3-y, wherein y has values of from 0 to 2, and ethyldichlorosilane (EtHSiCl.sub.2)in small amounts. Furthermore, various straight-chain and branched alkanes and alkenes having up to 9 carbon atoms are also formed as impurities. Typical reaction conditions and process fundamentals are discussed in Chemistry and Technology of Silicones, by Walter Noll, Academic Press, 1968. Section 2.2 and in Ullmann's Encyclopedia Of Industrial Chemistry, VCH Publishers, 1993, Vol. A24, pages 24-26. The above cited references are herein incorporated by reference.
The methyl chlorosilanes are separated by distillation and freed from impurities. However, some alkenes and alkanes cannot be completely removed in this manner due to their boiling point or because of the formation of azeotropic mixtures.
Adsorption processes are well known for separating and purifying less readily adsorbable components from feedstreams containing mixtures thereof with more readily adsorbable components.
Pressure swing adsorption processes generally involve passage of the feedstream through two or more adsorber beds containing molecular sieves or other adsorbents which selectively adsorb the more readily adsorbable components of the feedstream. The adsorbers are arranged to operate in sequence with suitable lines, valves, timers and the like so there is established an adsorption period during which the more readily adsorbable components of the feed stream are adsorbed on the molecular sieve or other adsorbent and a regeneration period during which the more readily adsorbable components are desorbed and purged from the adsorbent to regenerate it for reuse.
Such selective adsorption commonly occurs in the adsorber beds at an upper adsorption pressure, with the more readily adsorbable component thereafter being desorbed by pressure reduction to a lower desorption pressure. Such PSA processing is disclosed in U.S. Pat. No. 3,430,418 to Wagner and in U.S. Pat. No. 3,986,849 to Fuderer et al, wherein cycles based on the use of multi-bed systems are described in detail and are herein incorporated by reference. As is generally known and described in these patents, the contents of which are incorporated herein by reference as if set out in full, the PSA process is generally carried out in a sequential process cycle that includes each bed of the PSA system. In addition to the adsorption step, such cycles commonly include steps involving the release of void space gas from the product end of each bed in one or more cocurrent depressurization steps upon completion of the adsorption step. In these cycles, the released gas typically is employed for pressure equalization and for subsequent purge steps. The bed is thereafter countercurrently depressurized and often purged to desorb the more selectively adsorbed component of the feedstream from the adsorbent and to remove such gas from the feed end of the bed prior to the repressurization thereof to the adsorption pressure.
In the production of methyl chlorosilanes it is important to recover a significant portion of the methyl chloride from the direct synthesis product to minimize the loss of this valuable reactant and to control the release of this halocarbon to the atmosphere. Currently, methyl chloride is fractionated from the liquid portion of the direct synthesis product and absorbed and stripped from the vapor portion of the direct synthesis product. However, in the process of these fractionation steps, methyl chloride forms an azeotropic mixture with lower alkanes, particularly branched alkanes, such as isobutane, making further fractionation difficult and economically unattractive. Typically, this mixture is expelled from the process as a vent stream comprising gases such as methane, hydrogen, methyl chloride and nitrogen as well as residual amounts of alkanes. Because the vent stream may contain chlorinated compounds, it typically is incinerated. It is known to employ a pressure swing adsorption process with a silica gel or activated carbon adsorbent to concentrate the chlorinated hydrocarbons in the vent stream from a methyl chlorosilane reaction zone prior to incineration to remove gases which can be vented to the atmosphere or used as fuel. The concentrated chlorinated hydrocarbons such as methyl chloride and other residual materials can be subsequently incinerated. In this way the processing load on the incineration operation is reduced. Such processes do not address the recovery of methyl chloride for use by the methyl chlorosilane process. Others have employed such adsorbents as carbon for the adsorption of hydrocarbons and chlorinated hydrocarbons from a solvent mixture in air and subsequently desorbed the chlorinated hydrocarbon by heat desorption, as exemplified in U.S. Pat. No. 4,056,369. U.S. Pat. No. 4,713,4 13 discloses a process employing activated alumina to recover organic halides from a hydrocarbon solvent. Processes are sought which result in the economic recovery of the methyl chloride from these vent streams without the loss of methyl chloride or without the escape of the methyl chloride to the atmosphere.